Steam generation is necessary in heavy oil recovery operations. This is because in order to recover heavy oil from certain geologic formations, steam is required to increase the mobility of the sought after oil within the formation. In some heavy oil recovery operations, steam may be generated in once-through type steam generators (“OTSG's) or in packaged boilers. When OTSG's are used, high blowdown rates, often in the range of from about 20% to about 30% or thereabouts are encountered. Such blowdown rates lead to significant thermal and chemical treatment inefficiencies. And, such OTSG's often operate with a feedwater that has from about 2000 mg/L to about 8000 mg/L of total dissolved solids. In any event, such once through steam generators provides a low quality or wet steam, wherein about eighty percent (80%) quality steam is produced. In other words, the 80% quality steam is about 80% vapor, and about 20% liquid, by weight percent. The steam portion, or high pressure steam produced in the steam generators is injected via steam injection wells to fluidize oil in a geologic formation, such as oils in tar sands formations. The injected steam eventually condenses and an oil/water mixture results, and which mixture migrates through the formation, and is gathered at oil/water gathering wells, through which the oil/water mixture is pumped to the surface. Then, the sought-after oil is sent to an oil/water separator in which the oil product is separated from the water and recovered for sale. The produced water stream, after separation from the oil, is further de-oiled in a de-oiling process step. The de-oiled produced water stream is then further treated for reuse in a costly water treatment plant sub-system before it can be sent to the steam generators.
In one process known as the steam assisted gravity drainage heavy oil recovery process (the “SAGD” process), it is preferred that one hundred percent (100%) quality steam be provided for injection into wells (i.e., no liquid water is to be provided with the steam to be injected into the formation). However, conventional prior art water treatment techniques present a problem for the use of once through steam generators in such a process. That is because in order to produce 100% quality steam using a once-through type steam generator, a vapor-liquid separator is required to separate the liquid water from the steam. Then, the liquid blowdown recovered from the separator is typically flashed several times in a series of flash tanks to successively recover as series of lower pressure steam flows which may sometimes be utilized for other plant heating purposes. After the last flashing stage, a residual hot water final blowdown stream must then be handled, by recycle and/or disposal. The 100% quality steam is sent down the injection well and injected into the desired geologic formation. Fundamentally, though, conventional treatment processes for produced water used to generate steam in a once-through steam generator produces a boiler blowdown which is roughly twenty percent (20%) of the feedwater volume. This results in a waste brine stream with dissolved solids content that is about fivefold the dissolved solids content of the steam generator feedwater. Such waste brine stream must be disposed of by deep well injection, or if there is limited or no deep well capacity, by further concentrating the waste brine in a crystallizer.
Another method for generating the required 100% quality steam for use in the steam assisted gravity drainage process involves the use of boilers, which may be packaged factory built boilers of various types, or which may be field assembled boilers with mud and steam drums and water wall piping. Various methods can be used for preparing water of a sufficient quality to be utilized as feedwater to a packaged boiler. Still, residual wastewater streams are produced that require further concentration before disposal, such as by a crystallizer system.
In any event, in the recovery of heavy oil using a steam assisted gravity drainage (“SAGD”) or similar system utilizing steam injection methods, it is important to separate steam condensate from the recovered oil. The steam condensate or water phase must be treated and purified to meet specifications suitable for introduction of the recovered water into the steam producing boilers.
In some heavy oil recovery operations, evaporation based water treatment processes have been used for processing water from oil/water mixtures recovered from geologic formations. Suitable evaporation based water treatment processes are described in detail in (a) U.S. Pat. No. 6,733,636 B1, issued May 11, 2004, entitled Water Treatment Method for Heavy Oil Production, (b) U.S. Patent Application Publication No. US 2003/0127226, published Jun. 10, 2003, entitled Water Treatment Method for Heavy Oil Production, and (c) US Patent Application Publication No. US 2005/0022989, published Feb. 3, 2005, entitled Water Treatment Method for Heavy Oil Production, the disclosures of each of which are incorporated herein in their entirety by this reference. Additionally, a membrane based process followed by evaporation based treatment processes may be used. In any case, a relatively purified water stream is produced, and a relatively concentrated waste brine stream is produced. The waste brine stream contains dissolved and suspended solids of varying content, depending upon a myriad of factors. However, regardless of the precise waste brine stream composition, in many cases such waste brine cannot be discharged to a publicly owned water treatment plant, or injected into the earth via a deep well, due to various issues, such as regulations, unavailability of treatment facilities, or due to technical problems with deep well injection. Therefore it is common practice to treat the waste brine stream to separate the brine into an additional pure water stream and a dry solids product. The dry solids are usually sent to a landfill or buried in a production mine or pit. However, present methods of achieving such results are quite expensive, both in capital and operating cost.
By way of example, the condensate or produced water, after separation from the oil, may be pre-treated by physical-chemical methods and/or membrane processes, but may then be sent to an industrial evaporator, typically of the vertical tube falling film type. Often, boiler blow down and other wastes are combined with the produced water before feed to the evaporator. The evaporators are generally large, treating as much as 1000 US gallons per minute (227 m3/hour) or more, and discharging waste water brines of approximately seven to twenty percent (7-20%) total solids from after processing a feedwater in put stream having about zero point three percent to about five percent (0.3% to about 5%) total solids. The concentrated waste brine discharged from the evaporators is then typically sent to a forced circulation crystallizer for final reduction to solids. Such crystallizers, usually of the forced circulation type, crystallize all of the solids in the waste water brine. The resultant solids are typically removed using a filter press, although other devices such as centrifuges of various types could be utilized. Such technologies are analogous to the process equipment utilized in the production of sodium chloride, or common table salt.
There is, however, a problem with the prior art approach to concentration of solids in waste brines from heavy oil production operations as just described above. The condensate or produced water stream, even after oil separation, may contain varying amounts of organic chemical species of various types. In the residual circulating brine, certain dissolved organic substances build up in the brine as it is concentrated. Such brines are often quite high in petroleum components that are chemically extractable when tested. The presence of such petroleum components hinders both crystal formation and crystal separation from the mother liquor. As a result, the devices which have heretofore been utilized for crystal separation, such as centrifuges or filter presses, are sometimes plagued with plugging, as well as with failures of various operating parts.
In order to overcome the just mentioned crystallization and separation difficulties, other process changes and equipment additions have been tried. For example, in one approach, a forced circulation crystallizer is utilized as in the previously described practice. However the crystallizer is employed as a concentrator, and crystals are not separated from the brine. In such a design, the use of centrifuges or filter presses is eliminated. Brine of about fifty to about sixty percent (50-60%) total solids is typically discharged. The waste brine, at from about fifty to about sixty percent (50%-60%) total solids is then treated in a rotary dryer, which is direct fired with heated air. The heated, dry solids are then discharged. Such a drying process is extremely complicated, however, as well as capital intensive and complicated to operate. Further, many such dryers require a feed stream of about ninety percent (90%) or more solids by weight for continuous operation. Therefore a large recycle stream, which returns dried solids product back to the dryer feed stream, is required. Such practice necessitates a pug mill and recycle bucket conveyer system, for conditioning and handling of the dry solids recycle stream. Due to the high temperature and corrosive nature of the feed brine stream, expensive alloy materials, such as nickel-chromium-molybdenum alloys, are required materials of construction for the dryer. Needless to say, the entire system is very capital intensive.
In summary, when used in water treatment systems in heavy oil recovery operations, a wastewater crystallizer often encounters difficulty in completion of the drying and in extraction of a solids product, since inorganic salts that are normally crystallized easily are not easily generated by the crystallizer, evidently due to the presence of various “non-extractable organics” present in the waste brine stream that is to be concentrated. In other words, the easily removed organics components, especially the oily components, are separated and sent for final processing before marketing, but the separation process leaves behind various organic substances, including humic and fulvic acid compounds, that collectively interfere with crystal formation as the waste stream is in the final concentration stages, and which interfere with the separation of crystals from a circulating crystallizer brine.
It is clear that the development of a simpler, more cost effective approach to disposal of concentrated brines resulting from produced water treatment in heavy oil operations would be desirable. Thus, it can be appreciated that it would be advantageous to provide a treatment process which minimizes or eliminates the production of undesirable liquid waste streams, while minimizing the overall costs of constructing, operating, and maintaining a water treatment plant utilized for the support of heavy oil recovery operations.
The foregoing figures, being merely exemplary, contain various elements that may be present or omitted from actual embodiments which may be implemented, depending upon the circumstances. An attempt has been made to draw the figures in a way that illustrates at least those process elements that are significant for an understanding of the various embodiments and aspects of the invention. However, various other elements of a solidification process may be utilized in order to provide a versatile process for reliably solidifying the waste streams from a waste water treatment plant supporting heavy oil recovery operations, in accordance with the teachings hereof and the claims set forth hereinbelow.